Filtration of molten metal

ABSTRACT

In the production of light metal castings by manual methods molten metal is drawn in a ladle from a pool of metal within a filter floating freely in a body of molten metal in a holding furnace. The filter comprises a body made of a material which is not attacked by the molten metal and is provided with one or more apertures at or near the bottom end, these apertures being covered with filter material so that solid inclusions are removed from molten metal entering the filter body.

Unite States Patent 1191 Collins et al. 1 1 Apr. 24, 1973 54 FILTRATION0F MOLTEN METAL 763.325 6/1904 Roche ..210/242 2,913,117 11/1959 Gould....2l0/242 [75] Inventors: Donald Louis William Collins, Rug- 3 47 7311/19 9 swdisgoodm m 3 X by, Warwickshire; Brian Arthur Gil- 2,373,3204/1945 Lovell et al. .....210/69 X lett, Banbury, Oxfordshire, both of444,500 l/1891 Meyer ..2l0/242 X England Primary Examiner.lohn Adee [73]Assgnee' fl E 1 g g ig i Attorney-Robert S. Dunham, P. E. Henninger,Lester e Omred ana a W. Clark, Thomas F. Moran, Christopher C. Dunham[22] Filed: Mar. 29, 1971 and Robert Scobey [21] Appl. No.: 128,876 [57]ABSTRACT In the production of light metal castings by manual [30]Farelgn Apphcanon Pnonty Data methods molten metal is drawn in a ladlefrom a pool Apr. 1, 1970 Great Britain ..15,536/70 of metal thin afilter floating freely in a body of molten metal in a holding furnace.The filter comprises a [52] US. Cl ..210/69, 210/71 y m de of a ma erialwhich is not attacked by the [51] Int. Cl. ..B01d 35/28 molten m l n ipr ided wi h one or more aper- [58] Field of Search ..210/69, 71, 72,83, tures at or r the bottom these pe r ing 210/242 covered with filtermaterial so that solid inclusions are removed from molten metal enteringthe filter body. [56] References cued 4 Claims, 2 Drawing Figures UNITEDSTATES PATENTS 3,654,150 4/1972 Eccles ..2lO/69 Patented April 24, 19733,729,097

2 Sheets-Sheet 1 F/ -7 7- 1, PM

H m w 2 i WM) 6) 69 0 (MW Q m Patented A ril 24, 1973 3,729,097.

2 Sheets-Sheet 2 FILTRATION OF MOLTEN METAL The present inventionrelates to casting of light metals, particularly aluminum (includingaluminum alloys) but also of other metals having comparably low meltingpoints, such as magnesium and its alloys. In particular the presentinvention relates to a method of filtering molten metal in theproduction of castings.

The production of light metal castings, other than pressure diecastings, is essentially a manual operation, in which molten metal iswithdrawn from a holding furnace by means of a ladle. Large quantitiesof internal combustion engine pistons are produced by pouring moltenaluminum alloy by means of a hand-held ladle into permanent steelmoulds. In producing this class of gravity die-castings molten metal istaken from the holding furnace in a quantity measured only by theoperators eye and any surplus in'the ladle is returned to the holdingfurnace before refilling the ladle for the next casting operation.

Piston castings, for example, are machined. Any inclusions of hardaluminum oxide are very objectionable both for the wear imposed on thecutting tools during machining and for the likely wear on the cylindersurfaces from any hard oxide exposed on the cylindrical outer surface ofthe piston. In the production of aluminum alloy piston castings .bynormal production methods, a significant proportion of the castings arerejected for this reason.

, It is well known in the production of aluminum and aluminum alloyingots to filter the molten metal during the pouring of the metal intothe continuous casting mould. A glass cloth bag has been found to be aparticularly suitable medium for the removal of solids, such as aluminumoxide, in this context.

In the production of aluminum alloy castings the metal is melted in alarge capacity melting furnace from which it is transferred to a numberof small holding furnaces, from which one or occasionally more operatorsmay draw molten metal by means of a ladle. The aluminum oxide dross ismainly produced in the melting furnace and during pouring from themelting furnace into the holding furnace, but does not arise to anysignificant extent in the holding furnace. Such oxide dross as isproduced in the holding furnace is in the form of a very fine film.Filtration of the metal in the holding fur nace should therefore verylargely obviate the introduction of oxide, particularly in the form oflarge hard grains, into the castings.

It has already been proposed to incorporate filter fixtures of variouskinds into the holding furnace. Some forms of filter fixtures haverequired periodic vertical adjustment to take account of variation inthe metal level in the furnace or have retarded removal of metal fromthe furnace when the metal level is low. Such filter fixtures have thusreduced the rate of production attainable by the operator and have thusnot been generally adopted in the production of aluminum alloy castings.It has already been proposed to filter aluminum in a holding furnace bymeans of a crucible which is supported by a framework extending acrossthe mouth of the furnace. Metal enters the crucible through apertures inthe bottom and rises through a bed of granular filter material. Asalready stated the disadvantage of that type of filtering device is thatthe level of metal in the crucible is dependent upon the level of moltenmetal in the holding furnace with the result that a decrease in metallevel also results in a drop in the metal level in the crucible withresultant difficulty in withdrawal of molten metal in a ladle.

In another similar arrangement the crucible was held on a variablesupport for height adjustment. Since in such arrangement the supportextends downwardly into the furnace from above it causes obstruction inthe accessibility of the pool of metal in the filter to the ladle.

It has now been appreciated in accordance with the present inventionthat these objections can be overcome if the filter body is arranged asa free floating unit in the holding furnace. The most suitable materialfor the construction of a floating filter body for filtering moltenaluminum is plumbago (graphite) which is sub stantially unaffected bymolten aluminum and has an appropriate density, that is to say somewhatless than the density of molten aluminum. However any other materialwhich can resist molten aluminum and has a density in the range of 1.52.5 would be suitable for the purpose of filtering molten aluminum andindeed some material of lower density may be employed in somecircumstances.

In constructing a floating filter a number of considerations must beobserved. Thus the width and depth of the pool of molten metal withinthe filter must be sufficient to permit substantially unimpeded removalof metal in the ladle. Furthermore the height of the filter body abovethe metal level must be kept small so as to avoid obstruction of thehandling of the ladle by the operator. Additionally the filter must bearranged so that it is fairly stable and not easily overturned byaccidental contact with the ladle, which would result in the admissionof unfiltered metal into the pool of metal within the filter body. Ingeneral it is preferred that the mouth diameter of the mouth of thefilter body be in excess of its depth.

The invention is further described with reference to the accompanyingdrawings, wherein FIG. 1 illustrates one form of filter for carrying outthe method of the present invention.

FIG. 2 illustrates a second form of filter for carrying out the methodof the invention.

In both FIGS. l and 2 a body 1 of molten aluminum alloy is maintained ina holding furnace 2, which is heated by gas burners 3. For convenienceof illustration the holding furnace 2 is shown greatly reduced in sizeand in practical operation it would be substantially larger in diameterthan the filters 4 and 4'.

The filter 4 shown in FIG. 1 comprises a cylindrical body 5 having anoutwardly extending peripheral flange 6. A shallow groove 7 is formed inthe wall of the cylindrical body 5 near the bottom end of the filterbody 4. This permits a layer of glass cloth 8 to be secured across theopen bottom end of the body 5 by means of a glass binder thread 9.

The body 5 is formed of dense graphite of a density around 2.5 but maybe formed of other material which is substantially unaffected by moltenaluminum, provided that such material is less dense than moltenaluminum. The weight of the filter body can be increased by means of aniron ring embedded in the graphite so as to be protected from contact bymolten aluminum to sink the filter 4 further into the aluminum, but thisis not usually necessary.

in operation metal is removed from the pool of mo]- ten metal within thefilter 4 by means of a ladle 10. This reduces the level of the metal inthe filter and the body 5 rises in the body 1 of metal. Metal then risesthrough the glass cloth 8 to restore the level of the metal in thefilter body with that of the body 1. Any grains of oxide or other solidin the metal are removed by the filtering action of the glass cloth 8.As the filter body rises in the metal as the result of the removal of aladle full of molten metal, there is some tendency for the solid grainsto be shaken off the glass cloth and thus the filter has someself-cleaning tendency.

An alternative form of floating filter 4' is shown in FIG. 2. In thiscase the floating filter 4 takes the form of a graphite crucible with aseries of apertures 11 formed in the bottom of it. A bed 12 of granularfilter material is supported in the bottom of the filter and performsthe same function as the glass cloth 8. The bed 12 is preferably trappedbeneath in a perforated disc (not shown).

The floating filter of FIG. 1 is the preferred arrangement, both becausemetal removed in the ladle is more quickly replaced through the glasscloth and because the filter body is more inherently stable and is verydifficult to upset by contact with the ladle. The layer of glass clothis very easy to replace.

We claim 1. A method of filtering metal in the production of castingscomprising placing a freely floating filter member in a body of moltenmetal, said filter member having an open top and an imperforate sidewall formed of a material, which is substantially unaffected by themolten metal, while permitting molten metal to enter the filter memberat the bottom end thereof through a filter medium associated with saidbottom end, so that said molten metal rises from below the level of saidbody and only through said filter medium to form a pool in said member,said filter member allowing sufficient width and depth of said poolformed by said rise of filtered molten metal within said member, topermit substantially unimpeded removal of metal in a ladle, andwithdrawing molten metal through the top of said filter member byinserting a ladle into submergence in said pool and removing the filledladle through said top of the member, and thus causing further moltenmetal to rise into said filter member through said filter medium fromthe body of molten metal.

2. A method according to claim 1 in which molten aluminum is filtered bymeans of a filter member formed of graphite.

3. A method according to claim 2 in which the pool of molten metal isformed in essentially cylindrical shape by containment in said side wallof essentially cylindrical form, said molten metal being filtered byrising through glass cloth constituting said filter medium closing saidbottom end of the filter member.

4. A method according to claim 3 which includes stabilizing the floatingof said member by providing outward support in the molten metal, aroundthe mouth of the member, with a peripheral flange.

2. A method according to claim 1 in which molten aluminum is filtered bymeans of a filter member formed of graphite.
 3. A method according toclaim 2 in which the pool of molten metal is formed in essentiallycylindrical shape by containment in said side wall of essentiallycylindrical form, said molten metal being filtered by rising throughglass cloth constituting said filter medium closing said bottom end ofthe filter member.
 4. A method according to claim 3 which includesstabilizing the floating of said member by providing outward support inthe molten metal, around the mouth of the member, with a peripheralflange.